Palladium-catalyzed asymmetric [4 + 3] cycloaddition of methylene-trimethylenemethane: access to seven-membered exocyclic axially chiral allenes

A palladium-catalyzed asymmetric [4 + 3] cycloaddition of the methylene-trimethylenemethane donor with an azadiene has been developed, affording benzofuro[3,2-b]azepine-derived exocyclic chiral allene with control of axial and point chirality. The target compounds were generated in good to excellent yields and with high diastereoselectivities and enantioselectivities (up to >20 : 1 dr, 99% ee). Furthermore, this cycloaddition reaction could be efficiently scaled-up and several synthetic transformations were accomplished for the construction of useful chiral allenol and chiral spirocyclic derivatives.

Palladium-Catalyzed Asymmetric (3 + 2) Cycloaddition of 5-Allenyloxazolidine-2,4-Diones with Barbiturate-Derived Alkenes: Synthesis of Spirobarbiturate-γ-Lactams

The 5-allenyloxazolidine-2,4-diones had been synthesized as novel precursors of π-allyl palladium zwitterion and were applied in a palladium-catalyzed enantioselective (3 + 2) annulation by using barbiturate-derived alkenes as the reaction partner in the presence of an axially chiral phosphoramidite ligand. This reaction proceeded smoothly under mild reaction conditions, affording highly functionalized spirobarbiturate-γ-lactam derivatives in excellent yields along with high diastereo- and enantioselectivities. The scale-up reaction and further transformation of the product were also successful.

Asymmetric Syntheses of Enantioenriched 2,5-Disubstituted Pyrrolidines

C₂-Symmetrical scaffolds are privileged ligands in metal catalysis and are also widely used in organocatalysis. Among these, 2,5-disubstituted pyrrolidines hold a paramount importance, especially since they also find application in medicinal chemistry. This review highlights the stereoselective syntheses of these C₂-symmetrical nitrogen heterocycles. It includes synthetic strategies based on the use of the chiral pool as well as the more recent sequences designed following major achievements in asymmetric catalysis.

Fungicide-inspired precursors of π-allylpalladium intermediates for palladium-catalyzed decarboxylative cycloadditions

Inspired by a fungicide, we designed 5-vinyloxazolidine-2,4-diones as new precursors of π-allylpalladium zwitterionic intermediates and developed palladium-catalyzed asymmetric (5 + 3) cycloaddition with azomethine imines and (3 + 2) cycloaddition with 1,1-dicyanoalkenes. Both reactions proceeded smoothly under mild reaction conditions to produce various chiral heterocyclic compounds in high yields with excellent enantioselectivities. These results revealed that 5-vinyloxazolidine-2,4-diones were a type of suitable precursor for palladium catalysis and will find extensive applications in Pd-catalyzed reactions such as cycloaddition and allylic alkylation.

Cooperative Tertiary Amine/Palladium-Catalyzed Sequential [4 + 3] Cyclization/[1,3]-Rearrangement for Stereoselective Synthesis of Spiro [Methylenecyclopentane-1,3'-oxindolines]

A cooperative tertiary amine/palladium-catalyzed sequential reaction process, proceeding via a [4 + 3] cyclization of isatin-derived Morita-Baylis-Hillman Expansion (MBH) carbonates and tert-butyl 2-(hydroxymethyl)allyl carbonates followed by a [1,3]-rearrangement, has been found and developed. A range of structurally diverse spiro[methylene cyclopentane-1,3'-oxindolines] bearing two adjacent β,γ-acyl quaternary carbon stereocenters, which are difficult to obtain by conventional strategies, were obtained in good yields. Further synthetic utility of this protocol is highlighted by its excellent regio- and stereocontrol as well as the large-scale synthesis and diverse functional transformations of the synthetic compounds. Moreover, the control experiments probably established the plausible mechanism for this sequential [4 + 3] cyclization/[1,3]-rearrangement process.

Enantio-, Diastereo- and Regioselective Synthesis of Chiral Cyclic and Acyclic gem-Difluoromethylenes by Palladium-Catalyzed [4+2] Cycloaddition

gem-Difluoromethylene moieties are attractive in medicinal chemistry due to their ability to mimic other more ubiquitous functional groups. Thus, effective asymmetric methods for their construction are highly desirable, especially for the industrial production of chiral drugs. Using a Pd-catalyzed asymmetric [4+2] cycloaddition between substituted-2-alkylidenetrimethylene carbonates and gem-difluoroalkyl ketones, we were able to easily access chiral 1,3-dioxanes that contain a tetrasubstituted difluoroalkyl stereogenic center in cyclic and acyclic skeletons. A novel phosphoramidite ligand, which contains a bulky 1,1-dinaphthylmethanamino moiety, was developed to provide the products in high yield with excellent enantio-, diastereo-, and regioselectivity. Strikingly, the gem-difluoro substitution pattern promotes the reaction, and pentafluoroethylketone, an α,α-difluorinated β-ketoester, and a β-ketosulfone are suitable substrates for this method.

A new type of δ-vinylvalerolactone for palladium-catalyzed cycloaddition: synthesis of nine-membered heterocycles

In this paper, a new type of δ-vinylvalerolactone was designed and synthesized, and used as a new precursor in Pd-catalyzed [6+3] cycloaddition with azomethine imines, leading to nine-membered 1,2-dinitrogen-containing heterocycles in 77-98% yields with >20 : 1 d.r. These nine-membered ring-fused products were further transformed into unusual tetracyclic bridged-ring compounds without loss of the diastereoselectivities.

Pd-Catalyzed [4 + 2] cycloaddition of methylene cyclic carbamates with dihydropyrazolone-derived alkenes: synthesis of spiropyrazolones

In this paper, a palladium-catalyzed [4 + 2] cycloaddition of 5-methylene-1,3-oxazinan-2-ones with 4-arylidene-2,4-dihydro-3H-pyrazol-3-ones has been developed to produce spiropyrazolones in high yields with excellent diastereoselectivities in nearly all cases. The cycloaddition reaction was scaled-up without significant loss of yield, and its synthetic utility has been demonstrated by further transformations of the products. The reaction type of N-Ts cyclic carbamates under palladium catalysis was extended to include [4 + 2] cycloaddition for the first time.

Pd-Catalyzed Enantioselective (3+2)-Cycloaddition of Vinyl-Substituted Oxyallyl Carbonates with Isocyanates and Ketones

The vinyl-substituted oxyallyl carbonates were exploited as a new C,O-dipole for enantioselective Pd-catalyzed (3+2) cycloaddition. The corresponding oxyallyl-Pd species was weakly nucleophilic to react with activated carbonyl compounds, affording multisubstituted and enantioenriched oxazolidinones and 1,3-dioxolanes with a high degree of chemo- and stereoselectivity. The synthetic transformations of oxazolidinone product were carried out to build enantioenriched α-chiral aminoketone and epoxy derivatives.

Stereoselective synthesis of functionalized azepines via gold and palladium relay catalysis

We developed a cycloisomerization/asymmetric [4 + 3] cycloaddition cascade reaction via gold/palladium relay catalysis, furnishing enantioenriched furan-fused azepines efficiently.

Direct access to α,α-chlorofluoro arylketones via mild electrophilic heterohalogenation of arylalkynes

By employing a compatible system of selectfluor and NaCl, a new practical approach for direct access to aryl α,α-chlorofluoro arylketones via mild electrophilic heterohalogenation of arylalkynes is presented.

Stereoselective Synthesis of Acyclic Tetrasubstituted Alkenes from Anilines by Dearomatization and Trimethylenemethane Cycloaddition

A method for stereoselective construction of acyclic all-carbon tetrasubstituted alkenes through insertion of nitrile-substituted trimethylenemethane into the aryl C-N bond in anilines via an aromaticity destruction-reconstruction process is reported. The process involves dearomatization, azo-[3 + 2] TMM cycloaddition and aromatization-triggered rearrangement.

Combining palladium and ammonium halide catalysts for Morita-Baylis-Hillman carbonates of methyl vinyl ketone: from 1,4-carbodipoles to ion pairs

Here we report that Morita-Baylis-Hillman carbonates from diverse aldehydes and methyl vinyl ketones can be directly utilised as palladium-trimethylenemethane 1,4-carbodipole-type precursors, and both reactivity and enantioselectivity are finely regulated by adding a chiral ammonium halide as the ion-pair catalyst. The newly assembled intermediates, proposed to contain an electronically neutral π-allylpalladium halide complex and a reactive compact ion pair, efficiently undergo asymmetric [4 + 2] annulations with diverse activated alkenes or isatins, generally with high regio-, diastereo- and enantio-selectivity, and even switchable regiodivergent or diastereodivergent annulations can be well realised by tuning the substrate or catalyst assemblies. An array of control experiments, including UV/Vis absorption study and density functional theory calculations, are conducted to rationalise this new double activation mode combining a palladium complex and an ammonium halide as an ion-pair catalyst.

Palladium-catalyzed asymmetric allylic alkylation (AAA) with alkyl sulfones as nucleophiles

An efficient palladium-catalyzed AAA reaction with a simple α-sulfonyl carbon anion as nucleophiles is presented for the first time. Allyl fluorides are used as superior precursors for the generation of π-allyl complexes that upon ionization liberate fluoride anions for activation of silylated nucleophiles. With the unique bidentate diamidophosphite ligand ligated palladium as catalyst, the in situ generated α-sulfonyl carbon anion was quickly captured by the allylic intermediates, affording a series of chiral homo-allylic sulfones with high efficiency and selectivity. This work provides a mild in situ desilylation strategy to reveal nucleophilic carbon centers that could be used to overcome the pK_a limitation of “hard” nucleophiles in enantioselective transformations.

A variety of “hard” α-sulfonyl carbanions of aryl, heteroaryl and alkyl sulfones were successfully employed as nucleophiles in palladium-catalyzed asymmetric allylic alkylation with excellent enantioselectivities.

A Unified Catalytic Asymmetric (4+1) and (5+1) Annulation Strategy to Access Chiral Spirooxindole-Fused Oxacycles

A unified catalytic asymmetric (N+1) (N=4, 5) annulation reaction of oxindoles with bifunctional peroxides has been achieved in the presence of a chiral phase-transfer catalyst (PTC). This general strategy utilizes peroxides as unique bielectrophilic four- or five-atom synthons to participate in the C-C and the subsequent umpolung C-O bond-forming reactions with one-carbon unit nucleophiles, thus providing a distinct method to access the valuable chiral spirooxindole-tetrahydrofurans and -tetrahydropyrans with good yields and high enantioselectivities under mild conditions. DFT calculations were performed to rationalize the origin of high enantioselectivity. The gram-scale syntheses and synthetic utility of the resultant products were also demonstrated.

A Double Deprotonation Strategy for Cascade Annulations of Palladium-Trimethylenemethanes and Morita-Baylis-Hillman Carbonates to Construct Bicyclo[3.1.0]hexane Frameworks

Here we report that the chemoselective activation of Tsuji's 2-(cyanomethyl)allyl carbonates to generate the palladium-trimethylenemethane 1,3-dipoles via a deprotonation strategy can be realized in the presence of Morita-Baylis-Hillman carbonates from substantial activated ketones. The following S_N 2'-addition enables the formation of new 1,3-dipole species having an activated alkene moiety through a second deprotonation process, which then undergo cascade [1+2]/[3+2] annulations to furnish complex bicyclic [3.1.0]hexane frameworks having three contiguous quaternary stereogenic centers with good to excellent enantioselectivity. Moreover, by using benzoyl aldehyde-derived substrates, a [1+4]/[3+2] annulation sequence is similarly developed to produce fused cyclopenta[b]furan architectures.

Ligand-Controlled, Palladium-Catalyzed Asymmetric [4+4] and [2+4] Cycloadditions

Ligand-controlled, palladium-catalyzed asymmetric [4+4] and [2+4] cycloaddition reactions of benzofuran-derived azadienes have been developed. Taking advantage of chiral P,N-ligand (S,R_p)-PPFA, we obtained a variety of benzofuro[2,3-c][1,5] oxazocines in good yields with excellent enantioselectivities via [4+4] cycloaddition reactions. Employing chiral P,P-ligand (S)-Cl-MeO-BIPHEP, the chemo- and regioselectivities were switched to synthesize tetrahydropyran-fused spirocyclic compounds in good efficiency via [2+4] cycloaddition reactions.

α,β-Disubstituted CF3-Enones as a Trifluoromethyl Building Block: Regioselective Preparation of Totally Substituted 3-CF3-Pyrazoles

An efficient pathway toward a novel class of trifluoromethyl building blocks was elaborated. The reaction of α-CF₃-enamines with arylaldehydes resulted in direct synthesis of α,β-diaryl-CF₃-enones isolated in up to 93% yield as E-isomers. The possible reaction mechanism was proposed using the Zimmerman-Traxler model. The reaction of α,β-diaryl-CF₃-enones with hydrazines opens a novel pathway to trifluoromethylated pyrazolines. Oxidation of pyrazolines with DDQ opened access to totally regioselective preparation of 3-CF₃-pyrazoles isolated in high yield. Using this strategy, 4-arylated derivatives of known drugs Celebrex, Mavacoxib, and SC-560 can be synthesized.

Sulfone as a Transient Activating Group in the Palladium-Catalyzed Asymmetric [4 + 3] Cycloaddition of Trimethylenemethane Enabling the Enantioselective Synthesis of Fused Azepines

The palladium-catalyzed asymmetric [4 + 3] cycloaddition of a sulfonyl-trimethylenemethane (TMM) donor with azadienes furnished various sulfonyl-fused azepines with exclusive regioselectivities and excellent stereoselectivities (up to >20:1 dr, >99% ee) in high yields (up to 93%). Moreover, sulfone, serving as a transient activating group of the TMM species, can be easily removed from the cycloadducts to provide the structurally simple fused azepines with excellent enantioselectivities. This strategy demonstrates sulfonyl-TMM as an effective surrogate of naked TMM with high reactivity, exclusive regioselectivity, and excellent stereoselectivity.

Asymmetric hydrogenation of trifluoromethyl ketones: application in the synthesis of Odanacatib and LX-1031

The asymmetric hydrogenation of trifluoromethyl ketones via iridium catalysis to access chiral secondary 2,2,2-trifluoroethanols is presented. The key intermediates of Odanacatib and LX1301 were prepared with high yields and enantioselectivities.

Dialkyl Ether Formation at High-Valent Nickel

In this article, we investigated the I2-promoted cyclic dialkyl ether formation from 6-membered oxanickelacycles originally reported by Hillhouse. A detailed mechanistic investigation based on spectroscopic and crystallographic analysis revealed that a putative reductive elimination to forge C(sp3)-OC(sp3) using I2 might not be operative. We isolated a paramagnetic bimetallic NiIII intermediate featuring a unique Ni2(OR)2 (OR = alkoxide) diamond-like core complemented by a μ-iodo bridge between the two Ni centers, which remains stable at low temperatures, thus permitting its characterization by NMR, EPR, X-ray, and HRMS. At higher temperatures (>-10 °C), such bimetallic intermediate thermally decomposes to afford large amounts of elimination products together with iodoalkanols. Observation of the latter suggests that a C(sp3)-I bond reductive elimination occurs preferentially to any other challenging C-O bond reductive elimination. Formation of cyclized THF rings is then believed to occur through cyclization of an alcohol/alkoxide to the recently forged C(sp3)-I bond. The results of this article indicate that the use of F+ oxidants permits the challenging C(sp3)-OC(sp3) bond formation at a high-valent nickel center to proceed in good yields while minimizing deleterious elimination reactions. Preliminary investigations suggest the involvement of a high-valent bimetallic NiIII intermediate which rapidly extrudes the C-O bond product at remarkably low temperatures. The new set of conditions permitted the elusive synthesis of diethyl ether through reductive elimination, a remarkable feature currently beyond the scope of Ni.

Highly diastereoselective synthesis of 3-methylenetetrahydropyrans by palladium-catalyzed oxa-[4 + 2] cycloaddition of 2-alkenylbenzothiazoles

A highly diastereoselective synthesis of 3-methylenetetrahydropyrans via palladium-catalyzed oxa-[4 + 2] cycloaddition of 2-alkenylbenzothiazoles with allyl carbonates bearing a nucleophilic alcohol side chain is presented. This synthetic methodology tolerates a wide variety of 2-alkenylbenzothiazoles and afforded the desired 3-methylenetetrahydropyrans in good yields and excellent dr. In addition, further derivatizations resulted in new scaffolds, making them useful synthetic precursors.

Forging Odd-Membered Rings: Palladium-Catalyzed Asymmetric Cycloadditions of Trimethylenemethane

The catalytic asymmetric synthesis of complex molecules has been the focus of our research program for several decades because such strategies have significant utility for the construction of chiral building blocks for drug development as well as the total synthesis of natural products. Cycloaddition reactions are very powerful transformations in organic synthesis providing access to highly functionalized motifs from simple starting materials. In concert with this central interest, four decades ago, we reported the palladium-catalyzed trimethylenemethane (TMM) cycloaddition for forging odd-membered ring systems. In recent years, we focused our attention on the development of powerful ligand scaffolds which enable the preparation of valuable products with complete control of chemo-, regio-, diastereo-, and enantioselectivity, thereby addressing several limitations in the field of palladium-catalyzed asymmetric cycloadditions. The first section of this Account will outline the discovery of a new class of highly modular pyrrolidine-based phosphoramidite and diamidophosphite chiral ligands which facilitate [3 + 2] cycloadditions of TMM donors, opening a new area in asymmetric construction of five-membered rings.The formation of the Pd-TMM zwitterionic intermediates is driven by the unique charge distribution of the cationic π-allyl motif, in which the most electropositive central carbon stabilizes the neighboring carbanion generated by either desilylation or deprotonation. The second section of this Account summarizes the scope of cycloadditions between Pd-TMM zwitterionic intermediates generated via desilylation and a variety of electron-deficient acceptors to access cyclopentanes, pyrrolidines and tetrahydrofurans. This section also includes the use of nitrile-, vinyl-, alkynyl- and allene-substituted TMM donors to rapidly generate cycloadducts with high molecular complexity. The extension of this strategy to include [6 + 3] cycloadditions and dearomative processes will also be presented. The third section will discuss a new generation of TMM donors substituted with electron-withdrawing groups such as nitrile, benzophenone imine, trifluoromethyl, and phosphonate, where the Pd-TMM zwitterionic intermediates are generated via deprotonation of the acidic C-H bond adjacent to the π-allyl motif. This new strategy has enabled the synthesis of heterocycles with increased numbers of functional groups in highly asymmetric and atom-economic fashion.Throughout this Account, we will describe the implementation of these transformations toward the rapid assembly of drug candidates and the total synthesis of natural products such as (-)-marcfortine C. We will also give details of mechanistic studies regarding relevant intermediates within the catalytic cycles of the different strategies, which allowed us to better understand the origin of selectivity with various donors.

Diastereoselective Synthesis of 2,3,4-Trisubstituted Tetrahydrofurans via Thermally Reactive 1,5-Diene-tert-butyl Carbonates

We report that 3,3-dicyano-1,5-dienes bearing tert-butyl carbonates can be thermally converted to 2,3,4-trisubstituted tetrahydrofurans. The transformation relies on two thermally reactive functional groups, a 1,5-diene and a tert-butyl carbonate, that react cooperatively to yield the furan scaffolds by thermal Cope rearrangement, Boc deprotection, and oxy-Michael addition. Described herein is background related to the discovery, optimization, and scope of the key transformation and representative functional group interconversion chemistry for the tetrahydrofuran scaffolds.

Carbene-Catalyzed Enantioselective Decarboxylative Annulations to Access Dihydrobenzoxazinones and Quinolones

A direct decarboxylative strategy for the generation of aza-o-quinone methides (aza-o-QMs) by N-heterocyclic carbene (NHC) catalysis has been discovered and explored. This process requires no stoichiometric additives in contrast with current approaches. Aza-o-QMs react with trifluoromethyl ketones through a formal [4+2] manifold to access highly enantioenriched dihydrobenzoxazin-4-one products, which can be converted to dihydroquinolones through an interesting stereoretentive aza-Petasis-Ferrier rearrangement sequence. Complementary dispersion-corrected density functional theory (DFT) studies provided an accurate prediction of the reaction enantioselectivity and lend further insight to the origins of stereocontrol. Additionally, a computed potential energy surface around the major transition structure suggests a concerted asynchronous mechanism for the formal annulation.

Palladium-catalyzed asymmetric formal [3+2] cycloaddition of α-N-heterocyclic acrylates with vinyl epoxides for construction of isonucleoside analogs

A highly efficient Pd-catalyzed asymmetric formal [3+2] cycloaddition of α-N-heterocyclic acrylates with vinyl epoxides has been developed for the first time.